February 27, 2008

MICROCLIMATE AND TERRESTRIAL VEGETATION

Land area on earth is generally covered either by forests or grasses/crops. Both these types of vegetations create and maintain specific microclimates within the plant cover that has characteristic differences from the general environment/climate of the area. The plant cover of an area determines many characteristics of the microclimate. In turn, that particular microclimate may maintain the specific plant cover despite changes in the overall macroclimate of the region. This interdependence of microclimate and plant cover is particularly observed in some regions of the globe where ancient types of forests are still present although macroclimate of the region has changed and become unsuitable for those forest types e.g. rainforest on latosols. Deforestation in such areas destroys the fine balance of microclimate and vegetation. Consequently, regeneration of such forest becomes either impossible or extremely slow. General microclimatic effects of grass/crop cover and forest cover in an area are discussed below in brief.

Microclimate within short grass/crop cover
Generalized effects of green grass/crop cover about one metre or so high are as follows:

Temperature: In the early after noon, temperature maximum is just below the crown where maximum energy absorption is occurring. Temperature is usually lower near the soil surface where heat flows into the soil. During night time, the crop cools mainly by long-wave emission and by some continued transpiration producing a temperature minimum at about 2/3rd the height of the crop cover.. Under calm conditions, a temperature inversion may form just above the crop cover.Wind speed: The velocity of wind is minimum in the upper crop canopy where foliage is most dense. There is marked increase in the wind velocity above this level while it is only slightly increased below this level.
Water vapour: The concentration of water vapour is maximum at about 2/3rd of the crop height where canopy is most dense due to maximum diurnal evotranspiration rate there.CO2 concentration: Carbon dioxide is absorbed during the day for photosynthesis by the leaves. Therefore, its concentration is minimum at about 2/3rd of crop height where canopy is densest. During the night, CO2 concentration is minimum at that height because its absorption for photosynthesis stops while its evolution due to respiration by leaves continues.

Microclimate within forest cover
Microclimatic effects within a forest cover are explained in terms of:
Morphological characteristics: such as branching (bifurcation), growth periodicity (evergreen or deciduous), size, density, texture, orientation of leaves and height of trees;Plant coverage: reflected by the density and distribution of dominant life forms andStratification of vegetation: reflected in he number of strata, height of different strata and morphological characteristics of each stratum.
Different forest types have different spatial organizations and so their effects on microclimate. For example, tropical and temperate forests may be compared for important characteristics as below:

Generalized microclimatic effects within a forest cover may be described as below:

Modification of energy transfers

The canopy of a forest cover significantly changes the pattern of incoming and outgoing radiation that is reflected mainly as;

Change in albedo: Albedo of a coniferous forest is about 8-14%, of deciduous forest is about 12-18% while that of a savannah or woodland is even more.

Change in energy trapped: The type of plants, density of foliage and pattern of foliage orientation are important in changing the amount of energy trapped within the canopy and the percentage of energy reaching the ground within a forest cover. For example, dense Fagus sylatica forest traps 80% of incoming radiation within the top layer of trees and less than 5% reaches the ground. Such energy trapping is more pronounced on sunny days.

Change in absorption of short-wave radiation: Absorption and reflection of shortwave radiation depends somewhat on the density and characteristics of the trees. Generally more UV radiation is absorbed by forest canopy than infrared radiation. For example, in the tropical forests of Nigeria, forest floor receives only 7.6% of 0.5 μm radiation while 45.3% of <0.6 μm radiation.

Change in penetration of light: Penetration of solar radiation within a canopy generally obeys Bouguer-Lambert law I = Ioe-KL (where, I = radiation intensity on a horizontal plane within the canopy; Io = radiation intensity on a horizontal plane above the canopy; L = leaf area index & K = extinction coefficient)

The value of K is constant for a given species and is related to leaf chlorophyll content, canopy reflectivity and canopy architecture. The value shows inverse relation to the chlorophyll content and reflectivity of leaves and lies between 0.3 and 0.5 for grass type canopies and approaches 1.0 for nearly horizontal leaves. As a result of these factors, intensity of radiation generally decreases exponentially downwards within a canopy reaching very low values at the ground level.

In general, light penetration within a forest cover depends upon the type of trees, spacing, time of the year, age of plants, crown density and tree height. In some major types of evergreen forests, the amount of light reaching the ground level is as below:

Birch-Beach forest- 50-75%

Pine forest- 20-40%

Spruce & Fir forest- 10-25%

Tropical forest- 0.1-0.01%

In the deciduous forests, light penetration increases during leafless period.

Light penetration also depends on the age of trees as it controls both the crown cover and the height of trees. In a Pinus sylvestris forest in Germany, the amount of light reaching ground level was 50% at 1.3 years of tree age, only 7% at 20 years age and again 35% at 130 years of age.

Thus, the plant cover in a forest maintains a complex, temporally changing vertical stratification of downwards decreasing light intensities. This provides a variety of light intensity niches for the plants of different light intensity requirements within the forest cover.

Modification of air flow
A wind profile develops within the canopy of a plant cover develops due to steady-state boundary layer flow. It is logarithmic above the canopy while becomes exponential within the canopy. The zero plane displacement (D) depends on he height of plants. The roughness height (zo), a measure of community roughness is effectively the thickness of a laminar sub-layer through which individuala elements project. Its value is related to the height, variation and spacing of individual elements i.e. plants and their parts. In extrapolation downwards of the logarithmic curve, the zero velocity intercept lies at the height (D+zo). If canopy were rigid, it would have a constant value but in the canopy, variation of surface roughness depends on the leaf flutter, branch movement and leaf streamlining. These variations cause variations of zo with wind speed. The surface frictional characteristics are entirely specified by D and zo.

Generally, lateral air movement is lesser within a canopy than the outside. Even large variations in outside wind velocities do not affect airflow inside forest cover. The wind velocity within the canopy also does not show strong change during day and night but overall wind speed is higher during daytime due to convectional effects.

In temperate and tropical forests reduction in wind speed is different due to different canopy structure, vertical stratification, density of stand i.e. tree spacing and season. From outer edge towards deep into the forest, the outside wind velocity was reduced within an European temperate forest by 20-40% at 30 m, 50% at 60 m and 93% at 120 m. Inside a Brazilean evergreen forest, outside wind velocity of 2.2 m per second was reduced to 0.5 m per second at 100 m and to negligible at 1000 m. In this forest, the outside storm velocity of 28 m per second was reduced to 2 m per second some 11 km deep inside the forest.

Modification of air temperature
The forest cover influences the air temperature within it due to complex interaction amongst a variety of factors such as sheltering from sun, blanketing at night, heat loss by evotranpiration, reduction in wind speed, obstruction to vertical air flow etc.

During daytime heating of leaf canopy causes a convectional transfer of sensible heat and the air temperature within the upper canopy may be higher than above the canopy or below it. At night, the relationship is reversed because the layer of air in upper canopy is being cooled by contact with leaves that are losing heat both by radiation and slow transpiration.

High evotranspiration depresses daily maximum temperatures and causes lower mean monthly temperatures in tropical and temperate forests. In temperate forest at sea level, mean monthly temperature from the outside may reach 2.2o C in summer but only 0.1o C in winter. In very hot summers, the difference may be more than 2.8oC. In the forests that do not transpire greatly e.g. forteto oak maquis of Mediterranean, the high day temperatures within the woods may cause mean monthly temperatures to be higher than in the open.

Elevation (altitude) in the same climatic zone may affect the degree of temperature decrease in the temperate forest. At 1000 m altitude, lowering of mean temperatures in temperate forest may be twice that at sea level.

Vertical stratification structure modifies the thermal profile within a forest in complex ways. In tropical forests, dense canopy heats up much during the day and rapidly cools at night showing a greater diurnal temperature range than the lower strata. Daily maximum temperatures of second story are intermediate between those of the tree-tops and undergrowth. On the other hand, nocturnal minima are higher than either tree-tops or undergrowth because the second story is insulated by the air trapped both above and below it.

Modification of precipitation
The influence of forest cover on the precipitation is a complex problem. The amount of rainfall in an area mainly depends upon the water vapour brought from the oceans by the winds. In the regions of favourable conditions of atmospheric circulation, the plant cover increases the roughness of Earth’s surface. It also intensifies water exchange over the forest by adding substantial amounts of transpired water vapour to the oceanic water vapour. These two factors intensify vertical air currents and thus cause increase in total precipitation over the area than would be possible by oceanic water exchange only. Proportion of such local precipitation in the total precipitation has been estimated to be 44.75% in Asia, 29.58% in Europe and Africa, 40.5% in S. America, 39.0% in N. America and 39.0% in Australia.

At the microclimatic level, orographic effect of increased lifting and turbulence due to forest cover may be 1-3% in the temperate regions. In Germany, a 6% mean annual increase in precipitation has been reported after afforestation with greatest increase occurring during drier months. In the areas of heavy fogs, the forest canopy can filter the fog from air to an extent that the negative interception may occur resulting in increased precipitation within the forest cover. Winter rainfall outside a Eucalyptus forest in Australia was 50 c while inside the forest it was 60 cm.

Modification of humidity
Establishment of saturation vapour pressure and consequently, a humidity profile depends largely on the wind velocity profile within a plant cover. In addition, saturation vapour pressure profile also shows close correspondence with the temperature profile, both during day and night. Actual vapour pressure at night reaches almost saturation as air and the forest canopy are cooled through radiation and convection. Some water vapour is transferred through transpiration from the canopy. During daytime, upper canopy is air-heated through convection and water vapour pressure cure shifts much from the saturation curve. Deficit between the two increases downwards but at quite lower level, the actual vapour pressure curve inflects. Towards the bottom of canopy it again approaches saturation curve due to transpiration coupled with low air movement and low temperature there.

In general, humidity conditions are very much different within the plant cover than the outside mainly depending on evotranspiration characteristics of the cover. Evotranspiration depends mainly on the type of plant cover, density of plants, vertical stratification structure and temperature, time of the day and season. It generally increases with the density of vegetation and relative humidity within the cover may be 3-10% higher than the outside, especially during summers. Rainforests have high transpiration and, therefore, generally higher humidity within them. Within temperate forests the mean annual excess of relative humidity is 9.4% in beech, 8.6% in Picea abies, 7.9% in larch and 3.9% in Pinus sylvestris forests. In the tropical forests, there is complete saturation during night while humidity shows decrease with height during the day. Temperatures within the forests differ strikingly from the outside. The vapour pressure within an oak stand was found to be higher than outside for every month except December.

Modification of carbon-dioxide in air
A profile of atmospheric CO2 is found within a plant cover. This profile shows much diurnal variation due to photosynthetic uptake of CO2 during the day. The concentration of CO2 in the soil is very low and its use by plants is spatially and temporally very heterogeneous.

During the day, CO2-concentration decreases from upper canopy towards the ground. It reaches a maximum point near the middle of canopy. Below this point, CO2-concentration rapidly increases towards ground becoming equal to CO2-concentration outside the canopy at a level roughly corresponding with compensation light intensity. CO2-concentration reaches fairly high level at the soil surface. Thus the CO2-concentration profile is formed due to f photosynthetic depletion in upper canopy, equilibrium corresponding to compensation point lower in the canopy and addition of respiratory CO2 from soil microbes, shaded lower leaves and roots. During the night, CO2-concentration gradually increases towards ground level due to respiratory additions.

Plant cover and soil
Soils are formed by mixture of weathered rock material with the organic matter derived from decomposition of mostly plant litter. The role of plant cover in pedogenesis and determination of the soil type of an area is clear from the fact that all zonal soil types correspond to specific types of plant covers.

Brown forest soils
Very high productivity of broad-leaved summer forests plays considerable part in the pedogenesis of brown forest soils by maintaining quite high activity of soil microorganisms and sol fauna, particularly earthworms.

In North Europe, clearing of broad-leaved forests present on acid brown soils that had developed on siliceous parent material resulted in the establishment of Ericaceous heath lands. This hastened podsolization in those areas. These podsols are being maintained today by burning and felling of trees along with maintenance of heath-land. In absence of such interference, Calluna-Erica heath-land is easily replaced first by bracken (Pteridium aquilinum) and then by conifers. These plants bring back the podsol soil to acid brown soil.

In the absence of normal forest, the brown forest soils can be maintained under grass cover because much organic matter is returned to the soil by extensive root system. However, inorganic fertilizers are needed even then. If land is under crop cultivation, both inorganic and organic fertilizers are needed to maintain the soil.

Red & brown soils of arid subtropics
The soils in arid subtropics formerly had luxuriant vegetation of Quercus ilex and Pinus halpensis. However, overgrazing in the areas of brown soils on limestone in Europe resulted in sparse vegetation of low trees causing conversion of brown soils (Terra fusca) to red soils (Terra rosa). Brown soils in many areas still have comparatively better sclerophyllous cover. Afforestation on red soils protects them from summer solar insolation and changes them to brown soils again.

Other red and brown soils
While red brown soils develop under subtropical dry forests, red-yellow soils develop under subtropical forests and red laterite soils under tropical forsts. Brown and grey soils are developed under deciduous forests, grey-brown soils under semi-desert or desert scrub forests and chestnut brown soils under steppe.

The plant cover and the cycling of nutrients are intricately interlinked with each other in the ecosystem. Nutrient cycling and biomass normally reach equilibrium under climax conditions. External influences, particularly destruction of plant cover may break this cycling and disrupt this equilibrium causing deterioration of the environment. For example, in North Europe, destruction of deciduous forest cover by human activities during Late Stone Age and Bronze Age caused loss of nutrients from the upper layers of soil. The deteriorated soil caused establishment of heath-lands on them. These heath-lands are presently maintaining and are being maintained by reduced nutrient cycling from the upper layers of soils only. In the tropical areas, laterite soils (latosols) have very deep crusts of weathering and are greatly leached of nutrients. However, rain forests of ancient geological ages on these soils provide rich vegetation of great biomass in which most of the ecosystem nutrients are locked up. Despite poor nutrient availability in the soils in these tropical rain forests, such forests are presently being maintained in these areas only due to highly efficient nutrient cycling from the upper layers of the soil. The huge amount of plant organic matter from the vegetation falling on the soil decomposes and provides nutrients to the plants again.

In the soil under a forest cover, the absorption of several nutrients from the deeper layers initially reduces their availability but the return of these nutrients with fall of litter again increases and maintains their availability in the upper layers of the soil. This effect is particularly marked for Mg and Ca. The efficiency of nutrient cycling is greatest in rain forests followed by deciduous forests, coniferous forests and grasslands in that order. Coniferous forests return 50-100 kg/ha/yr of ash elements while deciduous forests may return 200-270 kg/ha/yr. The return of Ca in the rain forests is 200-300 kg/ha/yr while in deciduous forests is only 150 kg/ha/yr.

The soil structure is also greatly affected by the plant cover because roots of plants have a direct influence in maintaining the rhizosphere bacteria whose capsular slimes and gums stabilize the soil crumbs. Rhizosphere zone in the soil provides nearly ideal conditions for both aggregate formation and aggregate stabilization by incorporation of bacterially synthesized macromolecules. In the grassland cover, rapid aggregate promotion is certainly due to rapid and prolific root production of these plants.

The plant cover also influences soil fauna and consequently, the soil structure. In the forest mull soils, the plant cover provides litter that promotes and maintains rich earthworm population in the soil. In these soils, earthworms create pore space through voided casts that are stabilized initially by fungal growth and later by cementation with bacterially produced polysaccharide macromolecules.